(1) Field of the Invention
This invention relates to a radiographic apparatus for picking up fluoroscopic images of a subject, and more particularly to a radiographic apparatus having a radiation grid for removing scattered radiation occurring when radiation passes through the subject.
(2) Description of the Related Art
Conventionally, in order to prevent scattered X-rays (hereinafter called simply “scattered rays”) transmitted through a subject or patient from entering an X-ray detector, a medical X-ray fluoroscopic apparatus or X-ray CT (computed tomography) uses a grid (scattered radiation removing device) for removing the scattered rays. However, even if the grid is used, a false image is produced by the scattered rays passing through the grid, and a false image by absorbing foil strips constituting the grid. Particularly where a flat panel (two-dimensional) X-ray detector (FPD: Flat Panel Detector) with detecting elements arranged in a matrix form (two-dimensional matrix form) is used as the X-ray detector, a false image such as a moire pattern is produced due to a difference between the spacing of the absorbing foil strips of the grid and the pixel spacing of the FPD, besides the false image by the scattered rays. In order to reduce such false images, a false image correction is needed. In order not to produce such a moire pattern, a synchronous grid has been proposed recently, which grid has absorbing foil strips arranged parallel to either the rows or the columns of the detecting elements, and in number corresponding to an integral multiple of the pixel spacing of the FPD, and a correction method for use of this grid is also needed (see Japanese Unexamined Patent Publication No. 2002-257939, for example).
By way of correcting moire patterns, a method of image processing which includes smoothing, for example, is carried out nowadays. When false image correction is done to excess, the resolution of direct X-rays (hereinafter called simply “direct rays”) also tends to lower. Therefore, an attempt to reduce false images reliably through image processing will lower the resolution of direct rays, resulting in less clear patient images. Conversely, when greater importance is placed on the resolution of direct rays to obtain clear patient images, the false images will not be reduced through image processing, which constitutes what is called a trade-off between image processing and clearness. Thus, a perfect false image processing is difficult. Regarding the correction of the scattered rays remaining despite use of a grid, various methods have been proposed but these have disadvantages such as involving a time-consuming correcting arithmetic operation.
In connection with the correction method for use of a synchronous grid, Applicant herein has already proposed a method in which correction is carried out with respect to pixels shielded from direct rays by the absorbing foil strips, a distribution of scattered rays having passed through the grid is derived from the columns or rows of the shielded pixels, and signals of the other pixels are corrected based on the distribution. It has been proposed in the above method to set the distance between the grid and X-ray detector to an integral multiple of the height of the absorbing foil strips, and to set the position of the grid and the shape of the absorbing foil strips such that shadows of the absorbing foil strips fall only on certain pixel columns or pixel rows despite changes in the positions of a radiation emitting device such as an X-ray tube, the grid and the X-ray detector.
However, such conventional constructions have the following drawbacks.
When passing through the grid, most of the scattered rays are absorbed but parts thereof leave the grid without being absorbed. The manner of this passage is varied in different portions of the detecting plane of the X-ray detector, under the influence of a distortion in the arrangement of the absorbing foil strips. Specifically, the manner of passage of the scattered rays will be reflected on the X-ray detector. Thus, the influence of the scattered rays spreads over the entire X-ray detector.
In order to eliminate this influence, a pattern (striped pattern of the scattered rays) that appears in a patient image due to the manner of passage of the scattered rays being varied in different portions the X-ray detector may be stored beforehand as a rate of change map. This striped pattern of the scattered rays may be removed through the above image processing.
However, noise (statistical noise) not influenced by the scattered rays is superimposed on the above rate of change map. The striped pattern of the scattered rays is greatly changeable with the manner of distortion of the absorbing foil strips, and therefore its prediction is difficult. Thus, the rate of change map is acquired by actually applying X-rays to the X-ray detector with the grid attached thereto. At this time, there is no guarantee that the dose of X-rays reaching the detecting elements is the same for all the detecting elements, but certain variations will take place. These variations are superimposed on the rate of change map, causing the statistical noise. These variations occur also when the grid is not attached to the X-ray detector, and are independent of the above striped pattern of scattered rays.
To put it simply, the image processing for removing the striped pattern of scattered rays is carried out by placing the rate of change map on a fluoroscopic image. The statistical noise is superimposed on the rate of change map, apart from the striped pattern of scattered rays. When the rate of change map is applied to the image, pixel values of the image will be changed excessively by an amount corresponding to the statistical noise superimposed on the rate of change map. This causes granular noise to appear on the image.